This application relates to set accelerators for accelerating the hydration time of calcined gypsum. More specifically, it relates to a combination of calcium sulfate dihydrate and zinc sulfate as a gypsum set accelerator.
Gypsum, also known as set gypsum or calcium sulfate dihydrate, has long been a large volume commercial article of commerce in the building trades. Gypsum articles, including gypsum boards, molds, walls and the like, are prepared from an aqueous slurry of calcium sulfate hemihydrate. Calcium sulfate hemihydrate is also known as calcined gypsum, calcium sulfate semihydrate, calcium sulfate half-hydrate, stucco or plaster of paris. As the plaster hydrates, water reacts with calcium sulfate hemihydrate to form an interlocking matrix of calcium sulfate dihydrate crystals. Excess water must be purged, dried or allowed to evaporate from the gypsum-based product. The setting time of the slurry depends on a number of factors, including the age of the calcined gypsum, impurities in the calcined gypsum, surface area, pH, particle size and the temperature at the time of mixing.
Plasters are useful in a number of commercial applications. Molds are made of plaster for the manufacture of ceramic articles, such as crockery, dishes and bathroom fixtures. Plasters are also used for pottery, oil well cement, agricultural and general industrial uses. Plasters are also used for coating gypsum wallboard on interior walls where a plaster surface is desired, for example, to impart a textured surface. In addition, plaster is also used for setting-type joint compounds to treat the seams of drywall panels. Plaster mixtures, which form a machinable composition, are useful in creating industrial models or complex patterns on decorative columns or panels. In addition, plaster compositions are also used in the manufacture of wallboard, ceiling tiles and building fire blocks.
Where plaster compositions are utilized in all of the above applications, it is important to closely monitor and/or control the rate of hydration, which directly relates to when the plaster starts to set. Most all plaster uses require that the hydration rate, or set time, be controlled to a degree that allows for the efficient use of the plaster before it begins to set. Hydration rate and set time vary with each plaster composition. In the instance of a sprayable plaster, a spray apparatus employed to distribute the plaster slurry will often become clogged with solidifying calcium sulfate dihydrate crystals if the slurry sets more quickly than expected. Conversely, compositions, which set too slowly, may lose a desired form before they harden sufficiently to hold a shape. Thus, sprayable plaster compositions should ideally not set until they are emitted from the spray apparatus, but once sprayed, they should set quickly to retain the desired shape.
Accelerators for increasing the hydration rate, and for reducing the setting time of calcined gypsum, are well known in the art. The most common accelerator is calcium sulfate dihydrate that has been finely ground. When freshly prepared, it has high potency. However, when stored prior to use, it loses its effectiveness. U.S. Pat. No. 2,078,198, herein incorporated by reference, discloses improved accelerators comprising calcium sulfate dehydrate intermixed with sugar. This mixture renders the calcium sulfate dihydrate less subject to deterioration by aging.
Heating the co-ground sugar and calcium sulfate dihydrate mixture so that caramelized sugar forms a coating on the calcium sulfate dihydrate is disclosed in U.S. Pat. No. 3,573,947, herein incorporated by reference. The melted sugar coating further stabilizes the calcium sulfate dihydrate, reducing the effects of aging to a greater degree than the unheated sugar/dihydrate mixture. However, there is a limit to the amount of calcium sulfate dihydrate solids that can be added to a sprayable mixture for achieving an accelerated set. Once this upper concentration limit is reached, the concentration of the calcium sulfate dihydrate solids has not provided satisfactorily low set times, and also has been too viscous to pump through a spray applicator apparatus.
Zinc sulfate is used as an accelerator for ready-mixed setting type joint compound described in U.S. Pat. No. 5,746,822, which is herein incorporated by reference. No combination of zinc sulfate and calcium sulfate dihydrate solids used as an accelerator is contemplated in this reference. Also, zinc sulfate alone is not a sufficiently powerful accelerator for obtaining hydration rates and set times required for a sprayable plaster.
Aluminum sulfate is also known as an accelerator for reducing the set time of other plaster compositions. U.S. Pat. No. 3,984,269 discloses spraying an aluminum sulfate accelerator solution on the tape or at the joint of the seams in gypsum wallboard. When the accelerator comes into contact with the calcium sulfate hemihydrate composition, it expedites the setting reaction.
Aluminum sulfate hydrate has also been disclosed for use as an accelerator in U.S. Pat. No. 4,681,644, which is hereby incorporated by reference. The patent suggests the manufacture of gypsum board using an accelerator with multiple coatings. The accelerator is prepared by first coating freshly ground calcium sulfate dihydrate with sugar. Thereafter about 10-40% by weight of aluminum sulfate hydrate is ground with the sugar-coated dihydrate, based upon the weight of the total accelerator composition.
However, a disadvantage of the use of aluminum sulfate hydrate in accelerator compositions is that solutions of it in water are relatively corrosive, having a pH of approximately 1.0-1.7 (for 50% solutions). It has also been noted that in mixtures of aluminum sulfate and calcium sulfate dihydrate solids, at higher concentrations of aluminum sulfate, the aluminum sulfate does not uniformly disperse in the slurry, and migrates to the surface of the setting composition. This leaves an undesirable residue on the surface.
Yet another disadvantage of aluminum sulfate in accelerator compositions is that, when introduced into setting type joint compounds, it can cause undesirable foaming by reaction with calcium carbonate. In some applications, accelerator compositions including aluminum sulfate have resulted in set plaster having reduced strength.
It is an object of this invention to provide an improved set accelerator for at least one of accelerating the hydration rate and reducing the set time of plaster compositions in a predictable fashion.
It is another object of this invention to provide an improved set accelerator to accelerate the hydration rate of a tool or spray applied plaster without reducing the sprayability of the plaster slurry and/or the machinability of the set plaster composition.
It is yet another object of this invention to provide an improved in fluid form that is easily incorporated into the plaster slurry to accelerate the hydration rate of plaster mixtures and which has a relatively neutral pH.
The above-listed objects are met or exceeded by the present set accelerator for plaster which features a combination of calcium sulfate dihydrate and zinc sulfate material, which, when introduced into a slurry of calcium sulfate hemihydrate and water, causes it to have a relatively rapid hydration rate and/or set time, and to set rapidly and predictably.
More specifically, the present invention provides a set accelerator comprising a mixture of ground calcium sulfate dihydrate and zinc sulfate material for at least one of accelerating the hydration rate and reducing the set time of an aqueous slurry of calcium sulfate hemihydrate. Aluminum sulfate material may optimally be added when set times sufficient for sprayable plasters are required. A liquid set accelerator solution may be formed by the addition of water.
The present invention is useful for increasing or accelerating the hydration rate of a plaster composition. The set accelerator is particularly useful in sprayable plasters, where control of the hydration reaction is very important. Set times may be determined with great accuracy when the accelerator composition of the present invention is used. Prior to introduction of the present set accelerator, the hydration of the plaster composition to be sprayed proceeds slowly enough to prevent solids formation in the spray apparatus. The set accelerator is also able to accelerate hydration so that the plaster composition will hold its shape on a vertical surface without running or dripping. The accelerator of the present invention is able to control the hydration to achieve this balance. Optional coatings on the dihydrate may be used to maintain its activity over long periods of time.
Further, the set accelerator acts in such a way that it does not adversely affect the properties of the plaster, such as sprayability or machinability. Machinable plaster compositions must be relatively hard so that designs can be carved into the plaster or it can be precisely shaped with power tools. Addition of the optional aluminum sulfate hydrate material accelerates the hydration rate, and the set of the plaster composition even further so that it is suitable for use on vertical surfaces with a sprayable composition. In particular, this liquid set accelerator composition, when used in the spray apparatus of the preferred embodiment, mixes easily with the plaster composition to form a sprayable plaster composition that sets up in minutes.
The present invention also finds use in generating an especially useful set accelerator for use in ready mixed setting type joint compound as described in U.S. Pat. No. 5,746,822. Such plaster compositions require the use of a set initiator (accelerator) with a log K value greater than that for calcium ion in order to initiate setting. A liquid set accelerator would be preferred among users for ease and speed of incorporation of the set accelerator into the ready mixed setting type joint compound. Zinc sulfate is a preferred powdered material for set initiation, but is not when premixed with water. The present invention provides for a fluid suspension of the set accelerator that will provide efficient set initiation and acceleration, in a fluid form that is less corrosive than alum or ferric sulfate liquid accelerator solutions, while not adversely affecting the mixing and finish properties of the setting type joint compound.